People who are well educated may have a longer lifespan, with almost a year added for each year spent studying beyond school, according to a major study.

The study of the genes that underpin longevity also found that people who are overweight cut their life expectancy by two months for every extra kilogramme of weight they carry.

Researchers at the University of Edinburgh in the UK found that people who give up smoking, study for longer and are open to new experiences might expect to live longer.

"Our study has estimated the causal effect of lifestyle choices," said Peter Joshi, Chancellor's Fellow at the University of Edinburgh's Usher Institute.

"We found that, on average, smoking a pack a day reduces lifespan by seven years, whilst losing one kilogramme of weight will increase your lifespan by two months," said Joshi.

Researchers analysed genetic information from more than 600,000 people alongside records of their parents' lifespan.

Because people share half of their genetic information with each of their parents, the team was able to calculate the impact of various genes on life expectancy.

Lifestyle choices are influenced to a certain extent by our DNA - genes, for example, have been linked to increased alcohol consumption and addiction.

The researchers were able to work out which genes have the greatest influence on lifespan.

Their method was designed to rule out the chances that any observed associations could be caused by a separate, linked factor.

This enabled them to pinpoint exactly which lifestyle factors cause people to live longer, or shorter, lives.

They found that cigarette smoking and traits associated with lung cancer had the greatest impact on shortening lifespan.

For example, smoking a packet of cigarettes per day over a lifetime knocks an average of seven years off life expectancy, they calculated.

However, smokers who give up can eventually expect to live as long as somebody who has never smoked, according to the researchers.

Body fat and other factors linked to diabetes also have a negative influence on life expectancy.

The study, published in the journal Nature Communications, also identified two new DNA differences that affect lifespan. The first - in a gene that affects blood cholesterol levels - reduces lifespan by around eight months.

The second - in a gene linked to the immune system - adds around half a year to life expectancy.

The research used data from 25 separate population studies from Europe, Australia and North America.

"The power of big data and genetics allow us to compare the effect of different behaviours and diseases in terms of months and years of life lost or gained, and to distinguish between mere association and causal effect," said Professor Jim Wilson, of the University of Edinburgh's Usher Institute.

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